It is known to manufacture fibers of polymeric material by spinning, i.e., by extrusion of a molten polymer through a spinneret. It is also known to manufacture such fibers by film splitting or fibrillation. In the latter method the molten polymer is extruded through a linear or annular slit to form a film or sheet. Then the film is cooled below it fusion temperature, stretched to effect molecular orientation and subsequently subjected to a mechanical treatment to induce splitting or fibrillation of the stretched film.
The use of polypropylene as starting material in the film splitting method as mentioned above is well-known. Although it offers important advantages over other fiber-forming polymers, commercial acceptance of this method for the manufacture of fibers, particularly those in the low denier range, has in some cases been hampered by some properties which to a certain extent seemed inherent to fibers made by polypropylene film splitting, such as a rather high minimum average denier, a hard hand, and a gloss which gives them a "synthetic" appearance.
Blending polyolefinic starting material with synthetic rubbers to improve properties, other than those mentioned above, of fibers made from the polyolefinic material, such as tensile strength and crimp stability, has been suggested previously for high denier filaments. One would, therefore, expect that the use, as starting material, of a film consisting of a blend of polypropylene with a synthetic rubber might improve other properties of the fibers obtained by film splitting, but it has been found that most elastomers do not or only to a very limited extent fulfill this expectation.
In contrast with these generally disappointing results it has been found that the aforementioned disadvantages of film splitting to manufacture polypropylene derived fibers are avoided or at least considerably reduced by starting from polypropylene containing a certain amount of a styrene-butadiene block copolymer or polybutadiene yielding fibers with not only a mat surface when compared with fibers made from a film of unmodified polypropylene under the same process conditions, but also a lower average denier and a remarkably softer hand resembling that of wool or silk. Moreover, fibers made from film of such blends exhibit other attractive properties, such as a narrower denier range, improved resilience, and improved dyeability without appreciable loss in tenacity.